1. Field of the Invention
The present invention relates to an imaging apparatus, an imaging apparatus control method, and a computer program. More specifically, the present invention relates to an imaging apparatus, an imaging apparatus control method, and a computer program that make it possible to perform quick and accurate auto-focus processing for a target subject and prevent, in particular, an auto-focus control error due to a blink.
2. Description of the Related Art
Recently, an auto-focus (AF) mechanism that automatically takes the focus on a subject is mounted on many imaging apparatuses such as a still camera and a video camera. In photographing by general cameras, people are main subjects in a large number of cases. However, the people are present in various positions in photographed images and there are various compositions of the images. From the very beginning, a target subject may be present in a position where a range finding area in an imaging apparatus is not set. To cope with such a problem, it is proposed to recognize a face from an image in an imaging apparatus and set a range finding area in a position of the recognized face to make it possible to focus the imaging apparatus on the face regardless of a composition. For example, a focus control technique based on such face recognition is disclosed in JP-A-2003-107335. Focus control based on this face recognition makes it possible to perform appropriate focus control (focusing) in photographing of various compositions.
In photographing of a person, it is standard practice to target eyes for focusing. An apparatus that specifies eyes of a person as a range finding area and automatically performs focusing in a camera with an area of eyes of a subject as a reference is disclosed in JP-A-2005-128156.
As a method of focus control, a method of judging a level of contrast of imaging data acquired via a lens is known. This is a system for setting a specific area of a photographed image as a signal acquisition area for focus control (a spatial frequency extraction area), judging that the lens is more in focus as contrast of this specific area is higher and that the lens is out of focus when the contrast is low, and driving the lens to a position for increasing the contrast to adjust the focus. Such a focus control processing technique is disclosed in, for example, JP-A-10-213737.
Specifically, a method of extracting a high-frequency component of the specific area, generating integrated data of the high-frequency component extracted, and judging a level of contrast on the basis of the high-frequency component integrated data generated is applied to the method of focus control. In other words, a high-frequency component integrated value of the specific area is calculated for judging the level of the contrast and used as an evaluation value (an AF evaluation value). Auto-focus is realized by driving a focus lens to maximize the evaluation value. To perform the auto-focus, it is necessary to drive the lens with the evaluation value as a guideline. As a lens driving mechanism, for example, a voice coil motor is used.
However, in the auto-focus using an image contrast signal, if an area including eyes is set as the specific area to judge the level of the contrast, the contrast substantially changes when a person blinks during an auto-focus operation. As a result, it is difficult to perform accurate distance measurement and focus control.
The auto-focus using a contrast signal is on the premise that fluctuation in the contrast depends on only a change of focus. However, when the person blinks, a change in the contrast signal is caused simply by the blink. Thus, it is difficult to judge a contrast signal change due to movement of the focus and judge a focus position where the lens is focused.
A specific example will be explained with reference to FIG. 1. (A) and (B) in FIG. 1 are diagrams of an example in which a focus lens is moved from a close range side to an infinite side and an evaluation value indicating a level of contrast of a specific area including eyes of a person is measured. In (A) in FIG. 1, the person does not blink and, in (B) in FIG. 1, the person blinks.
In auto-focus processing in the case of (A) in FIG. 1 in which the person does not blink, fluctuation in contrast occurs depending on only a change in focus. Accurate focus control in which a peak position of an evaluation value corresponds to a focus position is performed. On the other hand, in the case of (B) in FIG. 1 in which the person blinks, a change in an image itself including the specific area including the eyes occurs, a change in contrast occurs on the basis of an image change due to a blink rather than a change in focus, and a peak position of an evaluation value is different from a focus position. Thus, accurate focus control is not performed.